Browsing by Subject "Ionization Chamber"
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Item Open Access Neutron Dosimetry of Mice Using Monoenergetic Neutron Beams(2011) Fallin, Brent AlanIn 2009 the researchers at Triangle Universities Nuclear Laboratory (TUNL) participated in a series of experiments with the Radiation Countermeasures Center of Research Excellence (RadCCORE). This thesis project is a component of the research done at TUNL that was partially supported by the RadCCORE collaboration. The primary goals of this work are: (1) to measure the neutron fluence (and hence the dose) from the standard neutron beam source at TUNL delivered to a small animal target to an accuracy of better than ± 10% and (2) to develop techniques for real time monitoring of the absolute dose delivered to small animal targets from neutron beam irradiation. These two projects are interconnected as the development of the real-time monitoring techniques depends on the results of the absolute fluence measurements.
Measuring the absolute neutron beam fluence necessitates the use of a reaction in which the neutron cross section is accurately known over the relevant energy range and a detection technique which is insensitive to gamma-rays or is capable of distinguishing gamma-rays from neutrons. In this work, neutron activation of aluminum and gold foils was used to make absolute measurements of the fast neutron (En ~ 10 MeV) fluence. Neutron activation of gold foils was also used to make a relative measurement of the thermal neutron fluence. The neutrons produced nuclear reactions in the foils, converting a small quantity of the stable atoms in the foils into radioactive ones which subsequently generate gamma-rays in their decay process. The activated foils are then removed from the beam and placed in front of a high-purity germanium (HPGe) detector that measures the energy spectrum of the gamma-rays emitted by the foil. By counting the number of gamma-rays detected over a set time, the incident neutron fluence at the foil location was determined using the known reaction cross sections. The measured neutron fluence was used to calculate the imparted dose to live mouse targets via the muscle tissue neutron kerma factors. Liquid and plastic scintillation detectors were also used to monitor the relative neutron flux in real time during the experiments. These relative detectors were subsequently calibrated using flux results obtained from the foil activation measurements and were used for real time dose monitoring.
The neutron beam produced at TUNL also has an intrinsic gamma component that adds to the dose received by a small animal target. The gamma contribution to imparted dose is generally taken to be around 10% or less for neutron beams created by linear accelerators utilizing the 2H(d,n)3He reaction, but no confirming measurements of this type have been performed at TUNL prior to this work. To verify this claim, an experiment was conducted to quantify the gamma-ray contribution to the target dose at several incident neutron energies and gas cell pressures.
The dosage from the mixed beam was measured using two ionization chambers that have different sensitivities to neutron and gamma radiation. The chambers were placed in the neutron beam, and the total charge induced in the ionization chamber by the mixed radiation field was monitored. The percent gamma-ray contribution to total target dose was calculated utilizing the procedures outlined in AAPM Report No. 7 and Attix.
Using the foil activation technique, the neutron fluence incident on target and dose delivered were measured to within ± 10%. The target dose estimated using the scintillation detectors was found to be accurate to within ± 20%. The results of the ion chamber measurements imply the gamma-ray component of the neutron beam at TUNL contributes less than 5% to the total target dose. Given the large difference in quality factors between gamma-rays (=1) and fast neutrons (~10), the contribution by gamma radiation to the total equivalent dose was determined to be negligible.
Item Open Access Physics Characterization of TLD-600 and TLD-700 and Acceptance Testing of New X-RAD 160 Biological X-Ray Irradiator(2013) Cao, YananProject 1: Physics characterization of TLD-600 and TLD-700
Purpose:
It is suggested that a pair of TLD-600 and TLD-700 can measure the exposure in neutron-photon mix fields. But the basic information of physics characterization of TLD-600 and 700 are not available. The purpose of this study was study the individual TLD variation and the energy dependence of TLD-600 and TLD-700.
Methods:
The individual calibration factors for 52 TLD-600 chips and 51 TLD-700 chips were determined under x-ray beams of 60 kVp, 80 kVp, 120 kVp, a mono-energetic 662 keV gamma beam of a Cs-137 source, and an Am-Be neutron beam (4.4 MeV). The individual calibration factor was calculated as the ratio of the group average response in uC/mR and the individual response in uC/mR. In addition, energy corrections factors for the individual calibration factors were determined, from each of the x-ray beams (60 kVp, 80 kVp, 120 kVp) to the 662 keV Cs-137 gamma beams.
Results:
For TLD-600, the range and relative standard deviation of the individual calibration factors are: 60 kVp (0.94003-1.0927, 3.5369%), 80 kVp (0.9395-1.0867, 3.0952%), 120 kVp (0.83403-1.0796, 4.5732%), 662 keV (0.80465-1.1926, 9.2515% ), AmBe (0.91740-0.94905, 3.0882% ); and the energy corrections factors relative to the 662 keV Cs-137 beams are: 60 kVp (1.2223), 80 kVp (1.1013), 120 kVp (1.0299).
For TLD-700 the range and relative standard deviation of the individual calibration factors are: 60 kVp (0.94351-1.0630, 2.6044%), 80 kVp (0.91690-1.0614, 2.6996%), 120 kVp (0.95697-1.0474, 2.3606%), 662 keV (0.91348-1.2270 , 4.2243%), AmBe (0.79330-1.2268 , 9.1577%); and the energy corrections factors relative to the 662 keV Cs-137 beams are: 60 kVp (1.0373), 80 kVp (0.97661), 120 kVp (0.88532).
Conclusion:
We have measured individual calibration factors and the average energy correction factors for photon beams and Am-Be neutron beams. Our results will be used in the future experiments and measurements with TLD-600 and TLD-700.
Project 2: Acceptance testing of new X-RAD 160 Biological X-Ray Irradiator
Purpose:
An X-RAD 160 Biological X-Ray Irradiator was recently installed at Duke University to serve as a key device for cellular radiobiology research. The purpose of this study is to perform acceptance testing on the new irradiator for operator radiation safety and irradiation specifications.
Methods:
The acceptance testing included tests of the following components: (1) Leakage radiation survey, (2) Half-value layer (beam quality), (3) Uniformity, (4) KVp accuracy, (5) Exposure at varying mA (linearity of mA), (6) Exposure at varying kVp, (7) Inverse square measurements, (8) Field size measurement, (9) Exposure constancy.
The irradiation parameters for each components of first round of acceptance testing performed on September 21, 2012 were: Leakage radiation survey (none, 160 kVp, 18 mA, 200s), Beam quality (40cm, 50-140 kVp in 10 kVp incensement, 1 mA, 10s, none), Uniformity (40cm, 160 kVp, 18 mA, 15s, F1), KVp accuracy (40cm, 50-150 kVp in 10 kVp incensement, 10 mA, 15s, none), Linearity of mA (40cm, 160 kVp, 2-18 mA, 15s, none), Inverse square measurements (20-63cm, 160 kVp, 1mA, 30s, none), Field size measurement (40cm, 160 kVp, 10 mA, 15s, none), Exposure constancy (40cm, 160 kVp, 18 mA, 20s, none).
The irradiation parameters for each components for each components of second round of acceptance testing performed on November 18, 2012 were: Beam quality (40cm, 35-150 kVp, 1 mA, 10s, F1) , KVp accuracy (40cm, 35-150 kVp, 1 mA, 10s, F1), Variation of kVp (40cm, 160 kVp, 18 mA, 30s, F1), Linearity of mA (40cm, 160 kVp, 1-18 mA, 30s, F1), Uniformity (40cm, 160 kVp, 18 mA, 30s, F1), Inverse square measurements (20-63cm, 160 kVp, 18 mA, 30s, F1).
Results:
The first round of acceptance testing performed on September 21, 2012 failed due to the fact that the measured exposure along the X-axis was significantly non-uniform; the exposure greatly decreases going in the left direction, which is a clear indication of un-corrected anode heel effect. After the X-ray tube was returned to the manufacturer, the beam was reconfigured by tilting the X-ray tube. Another round of acceptance testing was performed on December 18, 2012.
Conclusion:
The acceptance testing fulfilled the initial purpose. The machine is currently used normally In the following experiments; routine maintenance and quality assurance (QA) are required.